1. Field of the Invention
This invention is related to an air filter for removing smoke and dust generated in image recording such as in an electrophotographic or electrostatic recording apparatus. More particularly, this invention is related to an air filter used in the recording apparatus wherein a toner image is formed on a recording sheet and is fixed by a fixing means of a flash lamp type.
2. Description of the Prior Art
In an electrophotographic/electrostatic recording apparatus such as a printer, a copying machine and the like, a toner image is formed on a recording sheet and subsequently fixed by a fixing means.
In the electrophotographic recording apparatus, an electrostatic latent image is formed on a photosensitive drum by an optical means and developed by a developer, thereafter the latent image is transformed into a toner image on the recording sheet. In the electrostatic recording apparatus, the electrostatic latent image is formed electrostatically on a membrane by a stylus electrode and is transformed into the toner image on the recording sheet. However, the fixing means, in other words, fusing means is almost the same.
A fixing means comprising two heat rollers is generally used, whereby the recorded sheet is moved between two rollers and the toner image is fixed by heat under the pressure of the rollers. However, with increases in recording speed, a fixing means utilizing a method of flash lamp heating, has been introduced which does not use heat rollers. This method generates smoke during the fixing process and it becomes necessary to provide an air filter which removes smoke and other dust such as toner particles, from an exhaust duct between the fixing means and an exhaust blower.
When toner particles generally comprising dye and resin materials are fused in the fixing process, they evaporate into several kinds of gases, resulting in the forming of smoke.
In the prior art, the smoke thus generated is removed by the air filter. The air filter consists of one or preferably plural filter units, each being formed of glass fiber, synthetic fiber or the like and each having a different porosity. The filter units are stacked in cascade in the path of flowing exhaust air in the order of porosity, the one having the greatest porosity being located at an upstream side of the air flow.
FIGS. 1(a) and 1(b) show schematic cross sections of a laser printer in order to explain the principle of electrophotographic recording technology and explain where the air filter is installed. In FIG. 1(a), photosensitive drum 1, the surface thereof being coated with photosensitive material, is provided and the drum is rotatable in a direction of an arrow. Optical unit 2 including a laser source, a lens unit, and a reflector (these are not shown) irradiates light 4 onto the surface of drum 1 from one end to the other end thereof, whereby the light is controlled by a signal corresponding to the printing image from buffer unit 3 which is connected to a device such as a central processing unit or microcomputer.
Photosensitive drum 1 is electrostatically charged uniformly by corona charger 5 before the drum rotates to the exposure position of light 4 from optical unit 2. In this process, the charged surface of drum 1 forms an electrostatic latent image corresponding to the printing image. When the drum is further rotated, its surface is subject to developing unit 6 which contains toner particles, thereby the electrostatic latent image is transformed into a toner image. With further rotation of drum 1, the toner image is transferred to a recording sheet 7, which is fed from hopper 8 and transported to transferring unit 9. The surface of drum 1 is further rotated and exposed to discharger 10, and then the remaining toner particles are brushed off by cleaning brush 11 and recovered in collecting unit 12. Recording sheet 7 having the transferred toner image thereon is further moved to fixing unit 13, and stored in stacker 14.
The fundamental structure of FIG. 1(b) is almost the same as shown in FIG. 1(a), except that a cut sheet is used as a recording sheet. The reference numerals used in FIG. 1(b) show the same or like parts as shown in FIG. 1(a). Hoppers 8 store cut sheets of different sizes, and each cut sheet is moved along the arrows to transferring unit 9 and then to fixing unit 13 and finally stored in stacker 14.
Fixing unit 13 of a conventional heat roller type is utilized in an electrophotographic recording apparatus having a low printing speed. In this case, the air filter is not needed (in FIGS. 1(a), 1(b) the air filter is not used), it is sufficient to exhaust warmed air out of the apparatus. When a high speed of printing is required, a fixing unit of the flash lamp type is utilized and the air filter has to be . installed in the exhaust path to remove smoke generated in the fixing process.
In the prior art, the air filter is installed such as shown in FIGS. 2 or 3. FIG. 2 is disclosed in Japanese Patent Application Kokai No. 56-54467, 14.5.1981 by H. Ishikawa et al, and FIG. 3 is disclosed in Japanese Patent Application Kokai No. 58-83879, 19.5.1983 by Y. Ito et al. In FIG. 2, recording sheet 7 is exposed to fixing unit 13 comprising two flash lamps 21, and smoke is sucked into duct 23 by blower 22. Two air filters 15 are provided in cascade in the flow path of the air. In FIG. 3, cylindrical air filter 15 is provided above fixing unit 13 and radiator 24, and recording sheet 7 is coming into duct 23 through an opening on the right side thereof and running downward and vertically between fixing unit 13 and heat radiator 24. Minute smoke particles are captured in air filter 15.
However, in the electrophotographic/electrostatic recording apparatus, especially, utilizing the toner developer of dry type as explained above, a lot of toner particles are inherently floating in the ambient atmosphere. These toner particles, the size of which are usually greater than that of smoke particles, are also attracted to the air filter and clog up the filter openings increases resistance against air flow. Therefore filtering of the air filter deteriorates rapidly in a very short time.